The damaging effects of sunlight on human skin have been observed since time immemorial and many remedies have been proposed to protect the skin from this damage.
In general terms, harmful ultra-violet (UV) rays, particularly those originating from sunlight, which penetrate the upper atmosphere and reach the earth's surface, can be classified into:
i. the energy-rich UV-B rays (290-320nm wavelength) which possess an intense physiopathological activity on the skin; these are absorbed just above the dermis and they are responsible for erythema and skin pigmentation, and PA1 ii. UV-A rays (320-400nm wavelength) which penetrate deeper into the skin (to the dermis and beyond). Their energy is much lower and the photobiological effects they cause are much more long term in nature, for example, they accelerate skin ageing. PA1 x has a value of from 133 to 673, PA1 y has a value of from 25 to 0.25. PA1 a has a value of from 10 to 114 PA1 b has a value of from 0 to 49 PA1 x has a value of from 388 to 402 PA1 y has a value of from 5 to 0.75 PA1 a has the value 14 PA1 b has the value 13 PA1 x has the value 249 PA1 y has the value 1.25 PA1 i. Guinea pig stratum corneum is isolated as fine sheets from guinea pig skin and air dried. PA1 ii. A piece of the stratum corneum is applied to the outer surface of a 0.5 cm quartz cuvette using a drop of distilled water to seal the stratum corneum uniformly to the quartz surface. PA1 iii. The quarts cuvette carrying the piece of stratum corneum is placed in the light path of the spectrophotometer which for this purpose is fitted with a fluorescence cut-off filter. This filter eliminates the autofluorescence of the stratum corneum and filters out all transmissions above 400 nm. PA1 iv. The stratum corneum is scanned from 290 to 400 nm and the spectrum obtained is saved as the control. PA1 v. The cuvette with stratum corneum is removed from the spectrophotomete and the test material (i.e. sunscreen) is applied to the stratum corneum at the rate of 1.5 .parallel.l/cm.sup.2, in accordance with German DIN protocol, and rubbed uniformly across the entire surface of the skin using ht finger fitted with a finger stall. PA1 vi. The applied sunscreen material is allowed to stand for 5 minutes at room temperature (20.degree. C.) to enable it to dry, and then the sample is rescanned in the spectrophotometer as before from 290 to 400 nm. This spectrum is saved as the test spectrum. No spectral absorbance changes were observed with drying times between 2 to 15 minutes; the 5 minute drying time was therefore adopted as standard. PA1 vii. The control spectrum is subtracted form the test spectrum to provide the spectral absorbance of the test sample of sunscreen material and this absorbence is converted to transmission. PA1 viii. The in vitro Sun Protection Factor (SPF) is finally calculated form the transmission measurements as described by Diffey et al, in a paper entitled: "A new substrate to measure sunscreen protection factors throughout the ultra-violet spectrum" in J. Soc. Cosmet. Chem. 40, 127-133 (May/Jun. 1989); see especially page 130.
Certain synthetic organic substances (sunscreens) whose molecules absorb the harmful ultra-violet rays have been proposed for use in mitigating the deleterious effects of ultra-violet radiation. Examples include:
2-hydroxy-4-methoxy-4-methyl benzophenone also known as Benzophenone 10 (CTFA), and available from Ward Blenkinsop under the Trade Name UVISTAT 2211;
2-hydroxy-4-n-octyloxybenzophenone, also known as Benzophenone 12 (CTFA), and available from American Cyanamid under the Trade Name CYASORB UV 531;
2,2',4,4'-tetraheptoxybenzophenone, also known as Benzophenone 2 (CTFA), and available from BASF Chemical Co. under the Trade Name UVINUL D50;
4-t-butyl-4'-methoxydibenzoylmethane, also known as Butyl Methoxy Dibenzoyl Methane (CTFA), and available from Givaudan Coporation under the Trade Name PARSOL 1789; and
2-ethylhexyl-p-methoxy cinnamate, also known as Octyl Methoxycinnamate (CTFA), and available from Givaudan Corporation under the Trade Name PARSOL MCX.
Some of these substances absorb more effectively in UV-A range thereby providing filtering of UV radiation in this range, while others are more effective in the UV-B range.
A common problem exists, however, whatever the choice of synthetic organic sunscreen, for protection from whichever wavelength of ultra-violet radiation, and this is that physiological damage to the body can occur, following topical application of these sunscreens in quantities necessary to provide effective filtering of harmful ultra-violet radiation. Even those synthetic organic sunscreens that are believed to be safe to use in this way, necessarily have safety limits imposed, based on the quantity applied to the skin, which can result in only moderate to poor protection from harmful ultra-violet radiation.
Natural hydroxy anthracenic polyglycosides have been reported as sunscreens (Bader et al, Cosmetics & Toiletries (1981), 96 (10) 67) and the use of coffee oil as a sunscreen has been reported in French Patent 2 479 688. Similarly, extracts of jujube fruits (JP 84-227813) and aloe extract (U.S. Pat. No. 4,656,029) are stated to be useful in skin applications.
Natural extracts of aloe, frangula, senna and cascara containing anthraquinone derivatives have also been reported as useful sunscreen agents (Fox, Cosmetics & Toiletries (1987), 102 (3), 4I).
These prior observations suggesting use of the natural materials as sunscreen agents have generated only limited commercial use, and none is as effective as most of the aforementioned synthetic sunscreens.
We have now discovered a series of substituted 1,3-diketones, which can be isolated from natural sources, which have wide spectrum absorption properties spanning both the UV-A & UV-B ranges, and which can be employed as sunscreen agents in cosmetic compositions particularly suited for topical application to the skin or hair. These naturally occurring substituted 1,3-diketones can accordingly be used in place of synthetic UV-A & UV-B screening compounds in skin compositions, such as those, particuarly PARSOL 1789, referred to hereinbefore.